A. BNF notation for syntax

A. BNF notation for syntaxhttps://www.w3.org/Notation.html1. RULE NAMING--命名规则SPACE, TAB, CRLF, DIGIT, ALPHA, etc.--这些通用基本规则用的大写"<" and ">" --表示字段中是 < and >  <name> --必选项尖括号不使用,直接用name,用尖括号只是更明显，2. RULE1 / RULE2: ALTERNATIVES --二选一RULE1 | RULE2  -- RULE1和RULE2二选一3. (RULE1 RULE2): LOCAL ALTERNATIVES--局部可选项(elem (foo | bar) elem)--得到的是(elem foo elem)|(elem bar elem)二选一4. *RULE: REPETITION--规则重复*RULE--表示0个或者无穷个RULE== ""|RULE|(RULE RULE)|3*RULE1*RULE--表示至少一个RULE== RULE|(RULE RULE)|3*RULE1*2RULE--标识至少一个之多两个RULE==RULE|(RULE RULE)5. [RULE]: OPTIONAL--可选项[option]--表示1个或多个option ==1*option6. NRULE: SPECIFIC REPETITION--规则重复次数nRULE--规定重复次数n3option--这里option option option ==3*3option7. #RULE: LISTS--规则重复 逗号间隔#RULE--表示0个或者无穷个RULE，逗号间隔1#RULE--表示至少一个RULE,逗号间隔== RULE|(RULE "," RULE)|3#RULE==RULE *(","RULE)1#2RULE--标识至少一个之多两个RULE,逗号间隔==RULE|(RULE"," RULE)==RULE *1(","RULE)8. ; COMMENTS--注释;单行注释。类似C语的//修改补充：1)规则定义（编译原理）   NonTerminal ::= RuleOfTerminalsAndNonTerminals   非终结符::=终结符和非终结符规则2)路径中大量使用斜线/所以二选一用竖线|3)有时候需要特地布局，Cstring字符串中使用  \n换行符newline  \r回车键carriage—return  \t分隔符tab  \b空格backspace  \f换页   http://www.360doc.com/content/11/1029/16/5482098_160101381.shtml通用规则HTTP data are written in line format, ie. line breaks are significant.http数据写成一行，在Line breaks--换行规则  CrLf ::= \r\nStrings输出字符串中用到引号要用转义\"(反斜杠和英文下的引号)也可以用反斜杠和八进制的对应ascii码（八进制0开始）

NotationA. BNF notation for syntaxThis section has three parts:(a) a straight copy of a section of RFC #822Standard for ARPA Internet Text Messages, August 13, 1982,(b) changes and additions to (a),(c) a set of rules that we use everywhere and that are listed here once.(a) NOTATIONAL CONVENTIONSThis specification uses an augmented Backus-Naur Form (BNF) notation. The differences from standard BNF involve naming rules and indicating repetition and "local" alternatives.1. RULE NAMINGAngle brackets"<" and ">"are not used, in general. The name of a rule is simply the name itself, rather than<name>Quotation-marks enclose literal text (which may be upper and/or lower case). Certain basic rules are in uppercase, such as SPACE, TAB, CRLF, DIGIT, ALPHA, etc. Angle brackets are used in rule definitions, and in the rest of this document, whenever their presence will facilitate discerning the use of rule names.(Note for WWW: we never use them)2. RULE1 / RULE2: ALTERNATIVESElements separated by slash ("/") are alternatives. Therefore "foo / bar" will accept foo or bar. NOTE: this rule is changed to use the vertical bar character "|" instead of slash, since the syntax for directory paths uses slashes heavily.3. (RULE1 RULE2): LOCAL ALTERNATIVESElements enclosed in parentheses are treated as a single element. Thus, "(elem (foo | bar) elem)" allows the token sequences "elem foo elem" and "elem bar elem".4. *RULE: REPETITIONThe character "*" preceding an element indicates repetition. The full form is: <l>*<m>elementindicating at least l and at most m occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero; "1*element" requires at least one; and "1*2element" allows one or two.5. [RULE]: OPTIONALSquare brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".6. NRULE: SPECIFIC REPETITION"<n>(element)" is equivalent to"<n>*<n>(element)"that is, exactly n occurrences of (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.7. #RULE: LISTSA construct "#" is defined, similar to "*", as follows: <l>#<m>elementindicating at least l and at most m elements, each separated by one or more commas (","). This makes the usual form of lists very easy; a rule such as '(element *("," element))' can be shown as "1#element". Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is, "(element),,(element)" is permitted, but counts as only two elements. Therefore, where at least one element is required, at least one non-null element must be present. Default values are 0 and infinity so that "#(element)" allows any number, including zero; "1#element" requires at least one; and "1#2element" allows one or two.8. ; COMMENTSA semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel with the specifications.(b) Changes and additions2) a rule is written asNonTerminal ::= RuleOfTerminalsAndNonTerminals3) because the slash is heavily used in directory path names, the alternatives are separated by a vertical bar "|" rather than a slash.4) sometimes it is necessary to specify a layout character sequence, such as newline. We have here adopted the conventions used in C strings:\nnewline\rcarriage-return\ttab\bbackspace\fform feed(c) General rulesHTTP data are written in line format, ie. line breaks are significant.Line breaksCrLf ::= \r\nStrings:The purpose of a string is to allow any sequence of printable characters and the space to be transmitted. A string is a sequence of ASCII characters, written using C's notation for strings. Thus, a string is surrounded by double-quote characters and excludes control characters.If a double-quote character is used inside a string, it must appear as the sequence \" (backslash followed by double quote).A control character can be represented by a backslash followed by its ASCII sequence number in octal notation.The NUL is not representable.